1. Field of the Invention
This invention relates to air conditioning units and more particularly to systems providing stored energy for use during periods of peak electrical energy demand.
2. Description of the Related Art
Current air conditioning units having energy storing systems have had limited success due to several deficiencies. A major deficiency includes the reliance on water chillers which are practical only in large commercial buildings. In moderate or small-sized commercial buildings, no market acceptable solution is available. Prior art systems do not have a suitable method of providing direct cooling, that is, operating with the energy storage medium removed from the system and therefore have efficiency penalties. Further, current systems have no mechanism to adjust varying volume requirements for refrigerant during different operating modes. U.S. Pat. No. 4,735,064 issued to Harry Fischer, the applicant herein, is representative of these devices. The tank or container which is shown in the Fischer patent stores energy during off-peak periods, generally at night, supplies energy during peak periods, generally in late afternoon. Electrical companies encourage the use of off-peak electricity use by lowering their rates during this period and discourage the use of on-peak electricity through higher rates and higher demand penalties. However, even although the prior Fischer device operates to use energy during off-peak times and avoids energy use during on-peak periods, it fails to achieve high-efficiency during shoulder periods, that is, when conventional direct cooling is required and the energy storage devices are by-passed.
Shoulder hours are defined by the electrical utility company and are generally morning hours and late evening hours. Because certain standards are mandated, direct cooling of the system during shoulder hours must reach a high level of efficiency. As a result of this requirement, current systems fall short of the minimum government standards.
There are primarily two methods commonly followed to avoid high electrical demand during peak summer hours. One method is called load shedding in which compressors are shut down during peak periods and cooling is supplied by stored energy such as ice to provide cooling. The other practice is called load leveling in which a smaller compressor is operated continuously. During periods of low cooling demand, energy as ice is stored. During periods of moderate demand, the small compressor unit matches the load requirement. During periods of high demand when the small compressor cannot supply the needed energy, the capacity of the system is supplemented by the melting of ice to make up the difference.
The ice freezing period during low air conditioning demand may be as long as 12-14 hours, contrasting to the peak demand period which may be as short as 3 hours or as long as 10 hours.
Generally, the higher the evaporator temperature the greater the EER of the system. For the "direct cooling" mode, the evaporator temperature is typically 47.degree. F. In the ice freezing mode, in contrast, the evaporator temperature is typically 28.degree. F. This loss of efficiency for ice making is compensated for by lower electrical rates during off-peak hours. Saturday and Sunday generally have no peak hours because the commercial and industrial load is low.